The instant invention relates to the heat treatment of metals, and more particularly, to an apparatus for rapidly quenching substantially flat metal workpieces which have been heated during a heat treatment process.
It is well known in the field of metallurgy that certain metals undergo phase changes when they are heated and cooled within certain temperature ranges. It is also known that the rate at which a metal article is cooled can have a significant effect on the phase changes which the metal of the article undergoes as it is cooled, and that as a result, certain physical properties of the metal can often be controlled or altered to a significant degree by cooling the article at a rate which produces a selective phase change. It has been found that this phenomenon applies to articles made of many metals and metal alloys and that it is particularly applicable with respect to articles fabricated from various carbon steels. Specifically, it has been found that the durability, flexibility, and hardness of carbon steel items are substantially enhanced when the items are heat treated in a manner which produces a phase change, wherein the carbon steel is transformed from a structural form known as austenite to a form known as martensite. Further, since carbon steel is a relatively inexpensive and highly desirable construction material for many items such as hardware items, the effective heat treatment of carbon steel items has particular significance. While the specific characteristics of each particular steel composition are different, generally each steel composition has a specific hardening range, and generally in order to achieve the desired phase change in an item made from carbon steel, the article must be cooled through its entire hardening range in a fraction of a second. Since the hardening ranges of carbon steels generally encompass several hundred degrees fahrenheit, this means that a carbon steel article must be cooled at an extremely rapid rate to achieve the desired change to martensite. For example, when heat treating articles made of carbon steel 1075, the articles must be cooled from a temperature of approximately 1400.degree. F. to a temperature of approximately 1000.degree. F. in well under one second in order to achieve the correct phase change.
While various quenching apparatus have been heretofore available for rapidly cooling metal articles, at rates which produce phase changes in the materials from which the articles are constructed, a particular problem is presented when cooling substantially flat metal articles. In this connection, it has been found that many metals undergo dimensional changes as well as phase changes when they are passed through their respective hardening ranges and that frequently if a metal article is cooled non-uniformly, different portions of the article can undergo different dimensional changes. Hence, if a substantially flat metal article is cooled non-uniformly, certain portions of the article can undergo greater dimensional changes than other portions of the article, and as a result, the article can become warped. Further, it has been found that since the surface to mass ratio of a substantially flat article is greater in the edge portions thereof than it is in other portions of the article, the edge portions tend to naturally cool at a faster rate than other portions of the article. Hence, if the article is cooled by immersing it in cooling fluid, the article is cooled unevenly and a certain amount of warping generally results. Similar effects are experienced when substantially flat metal articles are cooled non-uniformly in other areas thereof. For this reason, in order to effectively quench a substantially flat metal article, the article should be uniformly cooled and substantially all of the heat which is withdrawn from the article should be withdrawn through the surfaces on the opposite faces thereof, and the amount of heat which is withdrawn through the edge of the article should be minimized. Further, the article should be cooled at a rate which produces the desired phase change in the metal from which the article is constructed.
The instant invention provides a quenching apparatus which can be effectively utilized for uniformly cooling substantially flat metal articles at extremely rapid rates. More specifically, the apparatus of the instant invention can be utilized for effectively cooling a substantially flat metal article at a rate which generally allows the article to be cooled through the entire hardening range of the metal from which it is constructed in a fraction of a second. For example, a substantially flat article made of carbon steel can generally be cooled from a temperature of approximately 1550.degree. to a temperature of approximately 1000.degree. in well under a second utilizing the apparatus of the instant invention. Further, the apparatus is operative for cooling a substantially flat metal article so that substantially all of the heat which is withdrawn from the article is withdrawn through the side faces thereof and very little heat is withdrawn from the edges of the article. Hence, the apparatus of the instant invention can be effectively utilized for cooling a substantially flat metal article in a heat treatment process to achieve a desired phase change in the material from which the article is constructed without causing the article to become warped.
The apparatus of the instant invention generally comprises a substantially flat first plate having a plurality of spaced apertures therein, an absorbent first wick element which overlies a surface of the first plate, means for supplying a liquid coolant to the wick element so that the coolant is absorbed by and dispersed in the wick element, and means for urging a heated workpiece into face to face pressurized engagement with the wick element to effect intimate contact between the workpiece and the coolant which is dispersed in the wick element. The coolant which is supplied to the wick element has a temperature and a vaporization temperature which cause at least a portion of the coolant to be substantially instantaneously vaporized upon contact thereof with the heated workpiece to effect cooling of the workpiece by means of the latent heat of vaporization of the coolant. The vaporized coolant which is produced as the workpiece is cooled escapes through the apertures in the plate to prevent the formation of pockets of vapor adjacent the surface of the workpiece which could retard the cooling operation and cause the workpiece to be cooled unevenly. Further, in the preferred embodiment of the apparatus, vacuum is applied to the apertures in the plate to enhance the withdrawal of vaporized coolant from the wick element. Preferably, the means for urging a workpiece into pressurized engagement with the wick element on the first plate comprises a second plate which is substantially parallel to the first plate and movable toward the first plate for urging the workpiece into pressurized engagement with the first plate wick element between the first and second plates. In one embodiment of the apparatus, the second plate is internally cooled and it is directly engageable with a workpiece to urge it into pressurized engagement with the wick element on the first plate. Since this embodiment only includes a wick element on the first plate, preferably this form of the invention is used for cooling relatively thin workpieces where a sufficient amount of heat can be withdrawn from a workpiece through one side surface thereof. In another embodiment of the apparatus, the second plate also has a plurality of apertures therethrough, a second wick element is provided overlying a surface of the second plate which is opposite the first plate and means is provided for supplying coolant to the second wick element. Since this embodiment of the apparatus is operative for cooling substantially flat workpieces between a pair of wick elements, it can be utilized for cooling workpieces having somewhat greater thicknesses where it is necessary to effect cooling from both side surfaces of the workpieces in order to reduce the temperatures thereof at the desired rates.
The plates of the apparatus of the instant invention are preferably substantially parallel and they can be either horizontally disposed or non-horizontally disposed. Further, it is contemplated that the means for supplying coolant to the plates will be embodied in several different forms. When the plates are non-horizontally disposed, the means for supplying coolant to at least one of the plates preferably comprises a weir which is disposed adjacent an upper edge of the respective plate and means for supplying coolant to the weir. Accordingly, during the operation of the apparatus, coolant spills from the weir onto the wick element so that it gravitates downwardly therethrough and is substantially uniformly dispersed therein. When the plates are horizontally disposed, the means for supplying coolant to one or both of the wick elements preferably communicates with the wick elements through coolant apertures which are preferably disposed in substantially uniform array on the respective plates. In both of these embodiments, however, the coolant which is utilized in the apparatus preferably comprises water because water has a relatively high latent heat of vaporization and it is instantaneously vaporized when it contacts a workpiece which is heated to a temperature in the temperature range required for heat treating metals. Water is also preferable for various other well known practical reasons, although it may be desirable in some instances to add various additives to water to inhibit rust and/or corrosion. In the preferred embodiment of the apparatus of the instant invention, means is also provided for positioning a workpiece between the plates after it has been heated but before it is quenched. Specifically, means is provided for positioning a workpiece so that it is interposed in substantially parallel spaced relation between the first and second plates and the wick elements thereon before the plates are moved together. Accordingly when the second plate operates to urge the workpiece into pressurized engagement with the wick element on the first plate, the workpiece is substantially parallel to the wick element on the first plate and the surface of the workpiece which faces the wick element on the first plate is uniformly moved into engagement therewith so that the workpiece is substantially uniformly cooled during the quenching process.
It is seen therefore that the instant invention provides an effective apparatus for quenching substantially flat metal workpieces during a heat treatment process. The apparatus utilizes one or more wick elements which are saturated with a coolant to effect extremely rapid cooling of a workpiece. Further, vapors which are produced during the quenching process escape from the wick elements through the apertures in the respective plates so that they do not impede the quenching process and in the preferred embodiment these vapors are withdrawn under vacuum. Further, because the apparatus is operative with a wick element which engages a side surface of a workpiece, substantially all of the cooling is uniformly effected through the side surface of the workpiece and only minimal cooling takes place through the exposed edges thereof, and hence, the workpiece is cooled uniformly and warping is avoided. Further, because of the unique manner in which the apparatus is operative to effect quenching, a workpiece can normally be cooled through the entire hardening range of the material from which it is constructed within a fraction of a second.
Accordingly, it is a primary object of the instant invention to provide an apparatus for quenching substantially flat metal workpieces in a heat treatment process.
Another object of the instant invention is to provide an effective apparatus for uniformly quenching a substantially flat carbon steel workpiece so that it can be cooled through its entire hardening range within a fraction of a second.
A still further object of the instant invention is to provide an effective apparatus for quenching a substantially flat metal workpiece, wherein cooling is effected through the use of a wick element which is saturated with a coolant.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.